breakfast ideas for 6 yr old

My son is 6/AudHD/level 1.

He’s underweight for his age because it’s hard to find foods he’ll eat, and because he’s on Ritalin.

Breakfast is our biggest struggle. He’s a big fruit eater, but that’s not helping him gain weight. He really likes super carb-heavy super sugary treats like beignets and donuts but I realistically can’t get out and buy those every morning.

He won’t eat healthier options like eggs, oatmeal, etc.

If I buy cereal he wants marshmallow cereal and picks out the marshmallows.

We’ve tried toast sticks and frozen waffles but he’s burnt out on them.

Any ideas?

Thanks!